Given the sacrosanct aura that's developed around pretty much everything to do with the Beatles over the past half-century, it seems unbelievable that anything related to the Fab Four might have ever gone missing through the decades.

But in coming up with a 50th anniversary, 4K digital restoration of the band's beloved feature film debut "A Hard Day's Night," technicians at the Criterion Collection and Janus Films had to work around missing chunks from the first and last reels of the original negative. Producer Giles Martin, son of the Beatles' original producer, George Martin, also had to compensate for a missing stereo master of their early single "She Loves You" and use the existing monaural recording as part of a new audio mix of the film's dialogue and soundtrack.

All of which serves as a reminder that in early 1964 when Beatlemania was exploding worldwide, musical immortality seemed in doubt for the four lads from Liverpool.

"It was never my dad's intention to be digging this up after 50 years," Martin said this week. "I know it was his view that there would be more Beatles projects coming along down the line, and that some other young act would find the Beatles' spark and the same [phenomenon] would be replicated. I think that was the case really until about 15 years ago. Now the Beatles have become this cultural phenomenon and they are stamped in history, and that hasn't washed off in any way."

The younger Martin's mission in creating a surround-sound mix for a low-budget, black-and-white film that originally was presented in monaural sound in theaters around the world "wasn't to be a modernized version of 'A Hard Day's Night.'"

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After an appearance on CBS' "The Ed Sullivan Show," the Beatles shot to fame and forever became one of the world's most well-known bands. Here's a look back at their U.S. debut.

"It's not as if I'm mixing 'Avatar,'" he said. "It still should sound like it's in 1964."

In fact, Martin said, "the advantage of 5.1 is that you can actually be more faithful to the mono. ... The film was in mono, and I found it weird that we would be listening to the Beatles talk and have it all come out of the center [channel], but then the band would play and the music would come out of the left and right speakers."

The restored version expands some of the sonic elements but keeps the Beatles' voices at center stage. "It makes for a more immersive environment," Martin said, also noting that for the DVD and Blu-ray home video versions released last week, viewers have the option of choosing between a fully monaural audio mix or the 5.1 surround version. The discs also include bonus features, including the documentary "Things They Said Today" and a commentary track drawn from interviews conducted by Beatles expert Martin Lewis for the 2002 DVD release of "A Hard Day's Night."

Clip: 'In Their Own Voices'

A clip from "In Their Own Voices," a video featurette that's among supplemental materials included with the Criterion Collection boxed set of "A Hard Day's Night" for the film's 50th anniversary.

A clip from "In Their Own Voices," a video featurette that's among supplemental materials included with the Criterion Collection boxed set of "A Hard Day's Night" for the film's 50th anniversary.

The film itself "has never looked this good in theaters," said Criterion Collection President Peter Becker, because "the prints made in 1964 were two or three generations away from the original 35mm negative."

"When you're working on the Beatles, it's really a double-edged sword," said Lee Kline, who headed Criterion's film restoration team that located the best existing sections of the missing original negative to use for the restoration. "You're working with things so many people are excited about, and something that's very important to people's hearts. You can't talk to Beatles fans without some of them overwhelming you with how excited they are, and you take that into consideration."

The tradition of translating popular music performers to the big screen was a spotty one before the Beatles came along, with movies often placing performers in awkward settings by directors who often had no feel for the exuberant energy of rock music.

"A Hard Day's Night" director Richard Lester and screenwriter Alun Owen avoided those pitfalls by channeling the Beatles' inherent personal charm and sense of humor into their script, and allowing John Lennon, Paul McCartney, George Harrison and Ringo Starr the opportunity to improvise many of their lines.

"In this case, the fifth Beatle was Richard Lester," Becker said, referring to the 82-year-old director who has given his approval to the new restoration. "They trusted him, from his work with Peter Sellers and Spike Milligan and the Goons, which they totally loved. The level of freedom you feel in the film, a lot of that is from some of the things Lester introduced."

Becker said his hope is that the wide release of "A Hard Day's Night" will in some way echo the shared international experience it created originally. ("A Hard Day's Night" opens Friday at select theaters around the country, including the Cinefamily in Los Angeles and the Laemmle Playhouse 7 in Pasadena, with a sneak preview Thursday night at the Cinefamily as part of the "Don't Knock the Rock" film festival.)

"There are ways to mess up a restoration," he said. "You can over-produce things, and over-process them to where they start to lose their shimmer, lose their grace, lose their energy. For this film, that would have been a complete tragedy. This is all about life and liberation and freedom. The Beatles are constantly breaking out of rooms, they are uncontainable in every way, which is how they were in life.

"Much of the humor in the film comes from people trying to get them to stay where they're supposed to be, and they're always running away to where they're not supposed to go," Becker said. "That freedom and freshness has to be there."